This invention relates to a lens motor drive device in an automatic focusing camera.
Automatic focusing cameras employ a variety of lens driving systems. In one of the systems, the lens is moved back and forth by an electric motor which is called "a lens motor". The lens motor is, in general, controlled by a servo control system so that a large current is supplied to the motor when the lens is set away from the focalization point where the lens is focused on an object to be photographed, and the current is decreased as the lens approaches the focalization point.
Torque required for turning the lens, which is a load applied to the lens motor, depends on various conditions. For instance, the torque depends on where and how the lens extending mechanism stops. When the ambient temperature is decreased, the load resistance is increased, and therefore it is necessary to increase the torque. Furthermore, the required torque depends on whether the lens is turned in one direction or in the opposite direction.
However, a conventional lens motor drive control system is not so designed that it can follow up the above-described variations of the required torque. Therefore, in the case when the lens has been stopped near the focalization point and a small current is supplied to the lens motor from the lens motor drive control system, the lens motor may not be started. Since, as described above, the current supplied to the lens motor is decreased as the lens comes near the focalization point, the lens may not be driven by the torque of the lens motor. Thus, sometimes the lens motor is stopped although the lens is not brought to the focalization position yet.